One Definition Rule (ODR)¶

ODR is a rule for C++ programs that forbids declarations of the entities with same name but by different C++ code. Better/exact description is out of the scope of this document, please visit the links below for details if needed.

As a brief overview you can’t do things like:

// Boo.hpp

class Foo {
  int a;
};

// Bar.hpp

class Foo {
  double a; // ODR violation, defined differently!
};

Though this code looks trivial and violation is obvious, there are scenarios when it’s no so easy to detect such kind of errors, e.g. see examples from Library Symbols section.